The present invention relates generally to scanning two-sided documents in a raster scanning system and, more particularly, to a gyricon platen cover to mitigate show-through of the back side of the document while the front side of the document is being scanned.
Electronic document scanners, copiers, and facsimile machines transform an optical image of an original document into an electric signal suitable for storing, displaying, printing or electronic transmission. A typical photographic copying machine is provided with a transparent platen which supports an original document during the copying process. The transparent platen allows the graphic information on the front side of the original document to be scanned by the optical system within the copying machine.
A platen cover is pivotably mounted by a suitable hinge system adjacent to the platen. The platen cover is swung up to an open position for placement of the document face down on the transparent platen to be copied or scanned by illumination from within the photocopy machine. The platen cover is swung down to a position covering the platen to hold and secure the original document on the transparent platen. The lower surface of the platen cover is the background for the document during illumination for copying or scanning. The platen cover is swung up to an open position for removal of the document from the platen. Alternately, the platen cover can remain down and the document fed between the platen and the platen cover by belt or other means and removed after illumination for copying and scanning by the same means.
The original document is illuminated by a high intensity light source for photocopying. Typical techniques for illuminating the document are scanning, which involves a relative movement between the original document and the source of illumination, or by a full-frame high intensity flash exposure of the original document while both the document and the light source remain stationary.
Light is reflected off an image on the front side of the original document onto a photosensitive medium, or an array of photosensitive sensor elements. In the former case of the photosensitive medium, the scanning system projects an image of the graphic information to the photosensitive medium thereby creating a latent electrostatic image of the graphic information on the photosensitive medium. The latent image is developed on the photosensitive medium by the application of colored marking material, which is transferred to another support material, usually plain paper, to form a photocopy of the graphic information on the original document. In the later case of the array of photosensitive sensor elements, the graphic image information received by the photosensitive sensor elements will be stored electronically for copying, printing or transmission.
In scanning, the scan of one side of a typical two-sided piece of paper may contain images from both sides of the page. Light shining on the front side of the original document being scanned is transmitted through the document and is reflected by the image on the back side. The image on the back side, whether black or in a color, whether solid or grey scale, produces varying amounts of light back to the front side. The high contrast image of the front side will be combined with a low contrast version of the mirror image from the back side of the page during the scanning. This low contrast image from the back side of the page is called “show-through”.
With only the scanned image from one side, there is no way of distinguishing between the low contrast “show-through” image from the back side and any low contrast image scanned from the front side of the page.
When the back side is scanned, the same occurs with a low contrast mirror image of the front side.
The platen cover typically has a high diffuse reflectance, white, lower surface, adjacent to the back side of the original document and the transparent platen, to provide a white background for a photocopy of an original document that does not fill the platen area to prevent black framing in copies of less than full-size original documents.
However the contrast of the white surface of the platen cover behind the document with the black image on the back side of the document is the cause of show-through when the front side of the document is being scanned.
Another problem is the lack of contrast between the white surface of the platen cover and the edges of the white or light colored original document which makes detection of the original document's edges difficult for the scanner. A pre-scan detection of the location of the original document's edges enables the scanner to provide electronic registration and electronic skew correction for the subsequent scanning of the original document. Moreover, the detection of the location of the input document's edges enables the digital scanner to provide automatic magnification selection. However, this edge detection depends upon the ability of the digital scanner to sense the difference in the diffuse reflectance between the original document's background and the surrounding platen cover surface.
Alternately, the platen cover can have a non-reflectance black lower surface, adjacent to the back side of an original document and the transparent platen, to provide a black background for a photocopy of the original document. The light that passes through the original document will be absorbed by the black surface of the platen cover substantially reducing a show-through image being scanned.
The platen cover with a black surface allows the unambiguous detection of the edges of a white or light colored document by the scanner for electronic registration, electronic skew correction and automatic magnification selection.
However, the black surface platen cover behind the original document may cause other problems during scanning such as dark borders around the original document, dark circles or other shapes where punch holes or perforations exist in the original document, and dark borders around multiple images such as multiple original documents on a single scan.
It is an object of the present invention to provide a platen cover with a black background behind the document and a white background surrounding the document to mitigate show-through of the back side of the document while the front side of the document is being scanning.
It is another object of the present invention to provide a platen cover with a black background for pre-scan detection of the location of the edges of an original document on the platen for electronic registration, electronic skew correction and automatic magnification selection by the scanner.